(a) Field of the Invention
The present invention relates to a method and an apparatus, both for producing ultra-low-density polyurethane foam having a flat top.
(b) Description of the Prior Art
Polyurethane form has been put to various uses. A variety of products, ranging from one having a low density to one having a high density, have been produced for various purposes.
The conventional method of adjusting the density of a polyurethane foam is to increase or decrease the amount of the foaming agent (usually, water) or the foaming promotor (a low-boiling point solvent, such as trichloromonofluoromethane or methylene chloride). To produce a low-density polyurethane foam, much water is used to enhance expansion ratio.
Foaming of polyurethane is achieved by the carbon dioxide gas generated by reacting organic isocyanate with water. In the conventional method, water must be used in a great amount in order to produce a low-density urethane foam. The use of water in large quantities, however, results in the following problems.
(1) The reaction between isocyanate and a large amount of water generates much heat, raising the internal temperature of the resultant foam. Consequently, the foam is likely to be scorched and discolored. Thus, it is necessary to add a scorch retardant.
(2) At most 6 to 7 parts by weight of water can be added to 100 parts by weight of polyol to produce a lowdensity polyurethane foam. Such a polyurethane foam is very likely to be scorched. In the worst case, the foam will cause a fire.
(3) Since a large amount of organic isocyanate, trichloromonofluoromethane, or the like must be used, the gas loss of the resultant foam is great, inevitably reducing the yield.
In the conventional method, a polyurethane foam is produced as is shown in FIG. 1. That is, first, polyurethane foam stock solution is poured into large foaming mold 1. Then, the solution is foamed, and the foamed product is cured, thereby producing a polyurethane foam slab 2. In this case, the inner surfaces of mold 1 are covered with mold-releasing paper (not shown). This method, called "batch" method, can be performed in a relatively small space. It is also advantageous in that a slab of any desired shape can be obtained by using a foaming tank of that shape.
The batch-type method, however, has a drawback. After the polyurethane foam stock solution is stirred and then poured into mold 1, the foaming proceeds, whereby the surface of the solution rises in mold 1. As the surface of the solution rises, the viscosity of the solution increases. Hence, the friction between the inner surfaces of mold 1 and the surfaces of the foaming slab, except for the top and bottom thereof, increases and hinders the foaming. Consequently, completely foamed slab 2 has a rounded top as is shown in FIG. 1; it cannot have a flat top. Inevitably, it is necessary to cut the top portion of slab 2 to provide a slab having a flat top. This means that the batch method requires an additional step of cutting a slab, and involves wasting of material (i.e., the rounded top portion of slab 2).